Method of preparing 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal

ABSTRACT

A process for preparing  1,4,4,5,5,6,6-heptafluoro-2-trifluoro-3-oxaheptan,7-diol polyformal by the following steps in order: 
     (1) reacting 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol with formaldehyde in 80-90% (w/w) sulfuric acid in the presence of methylene chloride to produce 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal; 
     (2) treating the reaction mixture with aqueous H 2  O 2  to destroy free formaldehyde; 
     (3) removing methylene chloride from the reaction mixture; 
     (4) extracting the product 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal from the reaction mixture with ether; 
     (5) agitating the ether extract with an aqueous solution of (a) H 2  O 2 , (b) KOH or NaOH, and (c) NaCl or KCl to convert hemiformal end groups into hydroxy end groups; and 
     (6) isolating the product 2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol polyformal.

BACKGROUND OF THE INVENTION

This invention relates to fluoropolymers and more particularly tocurable polyfluoro prepolymers.

Fluoropolymers are used as binders in high-density explosives andpropellants and in energetic compositions requiring a high degree ofthermal stability. For example, fluoropolymers are used extensively insuch compositions which are pressed. However, very few fluoropolymersexist which have functional groups such as hydroxy which are suitablefor curing and thus can be used as binders in castable or extrudablecompositions. Two known examples of such polymers are thefluoropolyethers FC2202 and L9019 made by the 3M Company. While usefulfor some purposes, these polymers are expensive and completelyfluorinated. The absence of hydrogen limits the compatibility of thesepolymers with conventional plasticizers, curing agents, and otherpolymers.

Horst G. Adolph and Judah M. Goldwasser in U.S. Pat. No. 4,740,628,entitled"2,4,4,5,5,6,6-Heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolPolyformal and Method of Preparation," disclose a polyfluoro prepolymerwhich is miscible with conventional plasticizers, curing agents, andother polymers. They disclose that2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal is useful as a block for preparing block-copolymers withnon-fluorinated polymers such as polyethylene glycol orpoly(4,4-dinitroheptane-1,7-diol). Moreover,2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal may be cured by conventional means such as commerciallyavailable polyisocyanates.

Adolph et al. disclose the preparation of2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal by the reaction of2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol withformaldehyde and concentrated sulfuric acid in the presence ofdichloromethane. The crude product is purified by treatment withhydrogen peroxide and by heating under vacuum to remove volatilecomponents. This procedure worked satisfactorily on a small laboratoryscale. However, when this procedure was used repeatedly and on a largerscale, it was found that the molecular weight was not reproducible andthat difficulties in separation of the organic and aqueous layers werefrequently encountered. A more reliable and facile process was thereforeneeded to permit preparation of the2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal on a larger scale.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide an improvedmethod of preparing2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal.

Another object of this invention is to provide a method of preparing2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal in better yield in large scale production operations.

A further object of this invention is to provide a method of producing2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal on a large scale with good molecular weight control.

These and other objects of this invention are accomplished by providing

a process for preparing4,4,5,4,5,6,6-heptafluoro-2trifluoromethyl-3-oxaheptane-1,7-diolpolyformal comprising the following steps in order:

(1) reacting2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol withformaldehyde in 80-90% (w/w) sulfuric acid in the presence of methylenechloride to produce2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal;

(2) treating the reaction mixture with aqueous H₂ O₂ to destroy freeformaldehyde in the reaction mixture;

(3) removing the methylene chloride from the reaction mixture;

(4) extracting the product2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal from the reaction mixture with ether;

(5) agitating the ether extract with an aqueous solution containing

(a) from about 10 to about 30 weight by weight percent of H₂ O₂,

(b) from about 1 to about 5 weight by weight percent of a base selectedfrom the group consisting of NaOH, KOH, and mixtures thereof, and

(c) from about 10 to about 25 weight by weight percent of alkali metalhalide salt selected from the group consisting of NaCl, KCl, andmixtures thereof

for from 3 to 5 hours to convert the hemiformal end groups into hydroxyend groups; and

(6) isolating the product2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Adolph and Goldwasser in U.S. Pat. No. 4,740,628, supra, disclose thepreparation of a novel fluoropolymer2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal by reacting2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol withformaldehyde in the presence of 80-90 percent (weight by weight)sulfuric acid and an organic solvent (e.g., methylene chloride). Theyteach that the molecular weight of the polymer product can be controlledby adjusting the diol to formaldehyde ratio, the amount andconcentration of sulfuric acid used, and the quantity of organic solventpresent during the reaction. The reaction temperature is preferably from-5° C. to 10° C. and more preferably from 0° C. to 5° C.

Although the process works well on a laboratory scale to produce thepolymer in the desired molecular weight range, scale up has causedproblems. First we have found that methylene chloride acts as anemulsifier between water and diethyl ether, making a necessaryextraction step difficult. Second, we have found that the averagemolecular weights of the polymer are often higher than expected and arenot reproducible.

As to the first problem, after the reaction has been run, the reactionmixture is now treated with aqueous H₂ O₂ (30% H₂ O₂ in water) todestroy most of the remaining free formaldehyde. The methylene chlorideis then removed. This can be done conveniently by vacuum distillation.For instance, in the example distillation at 55° C. under a pressure of100 torr worked well.

Next diethyl ether or a similar organic solvent is used to extract thepolymer and unreacted diol from the sulfuric acid.

According to the prior art method (U.S. Pat. No. 4,740,628, supra) theextract should be washed with an aqueous solution of base (NaOH, KOH,etc.), salt (NaCl, KCl, etc), and H₂ O₂ to destroy any remainingformaldehyde. However, we have found that in the scale up procedure merewashing is not enough. Formaldehyde still remains in the form ofhemiformal groups on the polymer CH₂ CF₂ CF₂ CF₂ OCF(CF₃)CH₂ OCH₂ OH.When the extract is heated to remove the ether, hemiformal groups reactwith hydroxyl groups on the polymer to form higher molecular weightpolymer units. Because the number of hemiformal groups varies from batchto batch, the average molecular weight of the polymer product will beunpredictable.

In the present process the ether extract is agitated (e.g., stirred)with the aqueous solution of H₂ O₂, salt (NaCl, KCl, etc), and base(NaOH, KOH, etc) for from 3 to 5 hours. This is done to assure thatsubstantially all of the hemiformal groups on the polymer are convertedto hydroxyl groups according to the reaction: ##STR1## The aqueoussolution used for this step preferably contains from about 10 to about30 (w/w) percent of H₂ O₂. It also preferably contains from about 1 toabout 5 (w/w) percent of a base which is NaOH, KOH, or mixtures thereof.The aqueous solution also preferably contains from about 10 to about 25(w/w) percent of a salt which is NaCl, KCl, or mixtures thereof.

The organic (diethyl ether) phase is separated and washed with brineuntil a constant pH of about 6.5 is achieved.

The diethyl ether is then removed by vacuum distillation (for example,50° C. at 10 torr). Next the unreacted2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol andits cyclic formal are removed from the polymer by vacuum distillation(for instance, 24 hours at 120° C. and 0.5 torr).

The general nature of the invention having been set forth, the followingexample is presented as a specific illustration thereof. It will beunderstood that the invention is not limited to this specific example,but is susceptible to various modifications that will be recognized byone of ordinary skill in the art.

EXAMPLE

Three Kg of2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol (3ML-9347) and 4080 mL of dry dichloromethane (4Å sieves) was added to a 20L 3-neck flask cooled in an ice bath. A static atmosphere of drynitrogen was maintained in the flask throughout the reaction. Withagitation, a solution 219.6 g of paraformaldehyde in 2430 g of 87.1%sulfuric acid was slowly added while maintaining a temperature of 3°-5°C. This addition took 200 min. The fluoropolymer reaction mixture wasagitated for 21 h at 20°-22° C. The reaction mixture was then agitatedwith 1500 mL of 30% hydrogen peroxide for 3 h at 20°-22° C. Thedichloromethane was distilled off at 55° C. and 100 torr; 10.3 L ofdiethyl ether was added to the flask. The ether solution was stirredvigorously with 4.8 L of a solution containing 3.1 % potassiumhydroxide, 15.6% sodium chloride and 18.7% hydrogen peroxide for 3 hoursat 20° C. The ether solution was washed three times with 1 L of 24%sodium chloride. The pH of these washes were 11.75, 6.5, and 6.5. Theether solution was dried by agitation for 24 h with 278 grams of silicagel (Kieselgel 60, 70-230 mesh). The etherfluoropolymer solution wasfiltered into an addition funnel and added to a 5 L 3-neck flask in a50° C. water bath. The ether was recovered for use in the next batch.The last of the ether was removed at 50° C. and 10 torr. This procedureyielded 2919 g of crude fluoropolymer, number average molecular weight=1800, containing unreacted monomer and its cyclic formal. The latterwere removed by heating for 24 h at approximately 120° C. and 0.5 torrand were collected in a trap cooled with dry ice.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A process for preparing2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal comprising the following steps in order:(1) reacting2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diol withformaldehyde in 80-90% (w/w) sulfuric acid in the presence of methylenechloride to produce2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal; (2) treating the reaction mixture with aqueous H₂ O₂ todestroy free formaldehyde in the reaction mixture; (3) removing themethylene chloride from the reaction mixture; (4) extracting the product2,4,4,5,5,6,6-heptafluoro-2-trifluoromethyl-3-oxaheptane-1,7-diolpolyformal from the reaction mixture with ether; (5) agitating the etherextract with an aqueous solution containing(a) from about 10 to about 30weight by weight percent of H₂ O₂, (b) from about 1 to about 5 weight byweight percent of a base selected from the group consisting of NaOH,KOH, and mixtures thereof, and (c) from about 10 to about 25 weight byweight percent of alkali metal halide salt selected from the groupconsisting of NaCl, KCl, and mixtures thereof for from 3 to 5 hours toconvert the hemiformal end groups into hydroxy end groups; and (6)isolating the product2,4,4,5,5,6,6-heptafluoro-2-trimethyl-3-oxaheptane-1,7-diol polyformal.2. The process of claim 1 wherein the reaction of step (1) takes placeunder anhydrous conditions.
 3. The process of claim 1 wherein themethylene chloride is removed by distillation in step (3).
 4. Theprocess of claim 3 wherein the methylene chloride is removed by vacuumdistillation.
 5. The process of claim 1 wherein diethyl ether is usedfor the extraction in step (4).
 6. The process of claim 1 wherein theagitation in step (5) is caused by stirring.